The present invention relates to a class of meteorological test and measurement instruments, or apparatus, for measuring the total net energy flux difference between naturally occurring incoming solar radiant energy flux and surface reflected short-wave energy flux, and/or emitted radiant terrestrial infrared energy flux. Specifically, the field of invention is a class of instruments known as net radiometers. Net radiometers are typically used to assess the total net energy balance between the incoming solar and down-welling terrestrial infrared energy flux, and the surface reflected solar and ground emitted infrared energy flux.
Net radiometers are an important instrument for global climate change research and agro-meteorology. For global climate change research, net radiometers are often deployed in glacial studies, where they are used to monitor the total net energy exchange over the ice sheet. Net radiometers are also commonly deployed on forest floors, within canopies, and above forest canopies, in combination, to study correlations between biological activity and net energy flux. For agro-meteorology, net radiometers are typically used in combination with other metrological instruments to measure loss of water in wetlands such as the Everglades, or to control irrigation on large farms.
Net radiometers measure the difference between total incoming radiant solar and down-welling terrestrial infrared energy flux, and the surface reflected solar and emitted terrestrial infrared energy flux. Net solar radiation is the difference between the incoming radiant solar energy flux and the surface reflected solar energy flux, extending from 2000-4000 nm in wavelength. Net infrared radiation is the difference between the down welling and surface emitted upwelling terrestrial infrared energy flux, extending from 4000-50,000 nm in wavelength. Total net radiation is the total net difference between to incoming solar and down-welling terrestrial infrared energy flux, and the surface reflected solar and emitted upwelling terrestrial infrared energy flux.
Typically in the daytime, the majority of net radiant energy contribution comes from incoming short-wave radiant energy from the sun. At night the majority of net radiant energy contribution typically comes from up-welling long-wave far infrared radiant energy from the ground. Short-wave radiant energy is generally defined as radiant energy in the ultraviolet, visible, and near-infrared wavelengths. The spectral range is approximately 200 to 4000 nm. Any radiant up-welling or down-welling energy with a spectral range of approximately 4000 nm to 50,000 nm is referred to as long-wave far infrared radiant energy.
Two examples of net radiometer types are four-absorber and two-absorber net radiometers. A two-absorber net radiometer includes a single pair of virtually identical thermal absorbers, one upward facing and the other downward facing. Each thermal absorber is thermally responsive across the short-wave radiant energy and long-wave far infrared radiant energy spectra. The upward facing thermal absorber absorbs incoming radiant solar and down-welling terrestrial infrared energy from the sky above, while the downward facing thermal absorber absorbs radiant solar and infrared energy either reflected or emitted from the ground.
A four-absorber net radiometer includes two pairs of absorbers. One pair is configured to respond exclusively to short-wave radiant solar energy flux, while the other pair is configured to respond exclusively to long-wave far infrared radiant energy flux. Typically, each absorber is configured to respond exclusively to the radiant solar energy flux signal is covered by a dome that filters out any long-wave far infrared energy flux, permitting only the radiant solar energy flux signal to reach the absorber surface. The domes are typically made of glass or other material opaque to long-wave far infrared radiant energy flux. Typically, each absorber configured to respond exclusively to long-wave far infrared radiant energy flux is covered with a solar blind filter which blocks the transmission of any radiant solar energy flux, permitting only the far infrared signal to reach the absorber surface. Optical lenses and domes also have an added benefit of shielding the thermal absorber surfaces from the elements. Each pair of absorbers has an upward facing and downward facing thermal absorber. While each pairs of absorbers are configured differently from each other, each absorber within a pair is configured virtually identical to the other absorber within the pair.
Unlike their four-absorber counterparts, many two-absorber net radiometers have absorbers that are typically uncovered and exposed to the elements. This creates a number of problems and creates potential for measurement error. One form of measurement error results from moisture deposition and retention on the absorber surfaces. Moisture retained on the absorber surface typically come in the form of rain, snow, or dew and frost. Water blocks the transmission of long-wave far infrared radiant energy flux from being transmitted to and from the absorber surface. The black surface of the thermal absorber is especially prone to the formation of dew. Some domeless two absorber net radiometers, such as the Kipp and Zonen NR Lite2 and Delta Ohm LP Net 07 models, attempt to solve this problem by coating the thermal absorber surfaces with a water repellent or hydrophobic coating such as Polytetrafluoroethylene (PTFE), which is often sold under the brand name Teflon. They also conically pitch the outward facing absorber surface in order to encourage water to roll off the surface. While these measures help reduce moisture retention, they often do not adequately remove moisture from the thermal absorber surface.
Another form of measurement error is caused by convective signal interference from wind blowing across the absorber surface, commonly referred to as wind sensitivity effect. Wind induced sensitivity error can manifest as either as positive or negative signal offset bias on the net radiometer output signal and resulting net energy measurement calculation. Increasing the mass of the absorber so that heat is more difficult to dissipate slows down the response time of the instrument and makes the instrument less sensitive to the effects of wind. Reduced instrument response time however has the unwanted effect of making the instrument less responsive, or non responsive, to any sudden change in signal, particularly for any rapidly occurring low level solar or infrared flux signal changes. A third form of measurement error, which is difficult to quantify, are inconsistencies in performance characteristics that vary from unit to unit. These are considered in the art as an inherent limitation in two-way net radiometer design, otherwise known as sensitivity asymmetry effect.
Improvements in the art are always desirable therefore there is a need for a two-absorber net radiometer were measurement errors caused by moisture retention on the instrument active absorber surfaces, wind sensitivity effect, and inconsistencies in performance characteristics due to absorber sensitivity asymmetry effect, are minimized.